1. Field of the Invention
The present invention relates to the field of generating light using semiconductor devices, which will be called LEDs in the following. The invention is particularly useful for generating white light by having an LED emitting light of a first spectrum and mixing this light with a second spectrum generated by partially converting the light having the first spectrum. This conversion will be called color conversion in the following.
2. Related Technology
The color of the emitted (mixed) light obviously depends on the ratio of converted light to light of the original first spectrum. This ratio in turn depends on the effective path length of the light of the first spectrum when travelling through a color conversion layer. The effective path length can be different for different emission angles, which results in an uneven color temperature when looking at the angular distribution of the far field pattern illuminated by an LED module. It further depends on the superposition of the respective radiation patterns that are different for light originating of the semiconductor emitter (close to lambertian shape) and the converted light (isotropic luminescence). The color variation in most conventional types of LEDs typically manifests itself as a cooler white in the middle with a warmer white or yellowish ring off axis in the light beam. This implicates that the geometry of the color conversion layer is a factor for the homogeneity of the color temperature of the LED module.
It is known that a geometrically well defined (so-called “conformal”) coating of the color conversion layer on the LED chip is improving the angular homogeneity of the color temperature.
U.S. Pat. No. 6,650,044 is directed on a device and a manufacturing method (Stenciling) resulting in a configuration of a phosphor layer that is substantially a conformal Coating on a Flip-chip-Geometry, having a substantially uniform thickness essentially with surfaces in parallel to the side planes of the flip chip LED.
U.S. Pat. No. 6,642,652 proposes an LED device uniformly covered by a Phosphor conversion layer (single or multiple layer) with substantially constant thickness (thickness deviates max. 10% from average) above the top surface and adjacent to the sides of the semiconductor stack. The conversion layer does not significantly increase the size of the source compared to the bare LED. It is also limited to the LED and does not cover submounts or reflectors or PCB by more than 100 μm.
U.S. Pat. No. 7,078,732 proposes a color conversion layer that is disposed directly on a semiconductor body and shows a substantially constant layer thickness throughout. In FIG. 3 of this prior art document a color conversion element is shown that is placed on top of a housing where the LED chip is placed in a recess. The void space is filled with a transparent material. The conversion layer has substantially constant thickness and exceeds the active region of the LED chip. In this example the layer is, however, not “directly” disposed on the semiconductor.
US 2006/0071223A1 teaches a color conversion layer that is purposefully structured to achieve a homogeneous color space over all viewing angles. The structure is described as having periodically changing thickness or recessed regions (at the edges of the chip).
U.S. Pat. No. 6,963,166 shows an LED lamp with a resin portion (conversion layer) that covers at least one side surface of the LED flip chip, and with a curved reflector separated from the side surface that is needed to obtain the desired angular homogeneity.
U.S. Pat. No. 6,635,363 shows an LED with phosphor layer whose thickness is proportional to the intensity of the emitted radiation of the underlying LED, to improve homogeneity of the emitted radiation.